The invention concerns a method for the separation of polar organic compounds, preferably lower aliphatic alcohols, from an aqueous solution. In accordance with this invention, it is done with a liquid/liquid extraction. The aqueous solution is a fermentation liquid, which contains the dissolved portions of the polar organic compounds. Hereinafter, this fermentation liquid is referred to as "mash".
Production of ethanol, for example, as a liquid, transportable energy source has recently become more important for various reasons. Because of the shortage of energy resources in the world, particularly oil, ethanol has gained increased recognition as one of the alternative energy sources, that is, as basic industrial raw material as well as fuel. Several countries have already committed their energy policy to include the utilization of ethanol.
The use of local agricultural raw materials has the economic advantage of saving foreign currency for those countries which do not control any fossilized energy resources. Another positive aspect is the contribution of plant carbon to the carbon cycle, that is the use of solar energy.
The industrial technical production of ethanol requires improved, more economical and, with respect to the energy balance, more promising production processes than those which now can be considered the level of technology, which includes the periodically operating (batch) method. This also includes the necessity to operate fermentation as well as recovery of the fermentation alcohol on a continuous basis. The latter is already in use with batch and/or semi-continuous fermentation.
It is known that during the biochemical conversion from sugar to ethanol and CO.sub.2 with the aid of yeast, ethanol production can be increased by limiting the ethanol concentration in the fermentation fluid.
We are familiar with various methods, which deal iwth the removal of ethanol from the mash during fermentation, either through binary or through ternary vacuum azeotropic distillation at fermentation temperature. All of these methods, however, are not economical for large-scale production because the creation of a sufficiently high vacuum requires a lot of energy due to the CO.sub.2, which forms and is dissolved in the mash, and because the necessary equipment has to have large dimensions.
With ternary azeotropic distillation, it is also important to consider the fact that a suitable azeotropic developer is hard to find, which can be used sensibly with the existing pressure conditions and which also does not unfavorably influence yeast activity.
If ethanol is produced on a large scale, it is accompanied by a high accumulation of an ethanol-free watery solution, which in the following is called "distiller's wash." The amount of distiller's wash, which flows off in the end, depends on the sugar content of the substratum solution, added to the fermention, and on the amount of dry substance in the fermention solution. The latter strongly influences the amount of energy necessary to boil down the distiller's wash. It also points out a possible way to sensibly use this waste product. The amount of dry substance in the mash can be increased by partially maintaining the watery fermentation fluid cycle.